The effect of geometrical nonuniformity on electrical characteristics of ZnO varistors with Al2O3 dopant

The Al₂O₃ dopants affect the grain growth. The ZnO average grain size and standard deviation decrease with increasing Al₂O₃ content. The microstructure of ZnO varistor is more uniform with the increase of Al₂O₃ . Al₂O₃ dopant decreases the pore ratio by the spinning effect of of ZnAl₂O₄ spinel. The Al ₂O₃ dopants can effectively improve the voltage ratio and affect the electrical characteristics of ZnO varistors. The sample A2 with 0.005 wt.% Al₂O₃ content has a minimum voltage ratio and the best nonlinearity coefficient in high current region. Microstructure property has direct relations to breakdown voltage and nonlinearity coefficient. With the decrease of average grain size and standard deviation, the breakdown voltage and the nonlinearity coefficient increase. The complex analysis technique is used to characterize the AC response in heterogeneous microstructure system. The depressed angle in the complex impedance plane indicates nonideal (non-Debye or Cole-Cole) relaxation and is related to the degree of nonuniformity existing in barrier levels within each junctions